1. Field of the Invention
The present invention relates to methods and apparatus for enhancing reception of transmitted signals and, more particularly, to techniques for synchronously combining transmissions from plural transmitters at a distant receiver to extend range performance.
2. Description of the Related Art
One factor affecting range performance in wireless communication systems is the power with which signals are transmitted. Generally, the strength of a received signal is proportional to the transmit power and inversely proportional to the square of the range between the transmitter and receiver. At a given transmit power, as the range between a transmitting device and a receiving device increases, the signal strength at the receiving device becomes increasingly attenuated, eventually prohibiting reception. Range performance improves with increasing transmit power; conversely, lower transmit power reduces the maximum range at which transmitted signals can be detected.
Unfortunately, there are a number of circumstances in which transmit power is limited by equipment capabilities, operational requirements or both. For example, with mobile communication devices which rely on battery power supplies, the maximum transmit power may be limited by design to achieve a tradeoff between operating range and battery power consumption. Other equipment cost or performance considerations may dictate transmit power capabilities that limit operational range or limit system performance under harsh operating conditions.
In certain circumstances, a need may exist to minimize RF emissions. In military contexts, particularly in battlefield situations, minimizing transmit power reduces the likelihood of signal detection by hostile parties, thereby preventing the transmitter's position from being compromised. For example, in a situation where a small team of radio-equipped personnel is located within a hostile region and must communicate with a distant device, for any single radio to communicate with the distant device, considerable transmit power would be required, resulting in a significant opportunity for detection by hostile forces. Moreover, the transmit power level required to communicate over significant distances would likely drain the radio's battery more rapidly than desired.
In other contexts, low transmit power levels may be advantageous or required to minimizing interference with other devices, particularly in high bandwidth usage situations, such as with wireless telephony. In general, it would be advantageous in a variety of applications to achieve certain range performance with reduced transmit power levels or, conversely, to increase range performance without increasing transmit power.
Operational parameters other than transmit power can be optimized for better range performance under certain conditions. For example, antenna gain may be increased using a directional antenna, or more sophisticated receiver schemes can be employed. In some cases, the problem of limited range performance (or, equivalently, limited transmit power) may be overcome using repeaters to boost the signal power at an intermediate location between the source transmitter and destination receiver. However, each of these solutions has drawbacks, such as increased size, cost and circuit complexity, overall system complexity, and increased energy requirements. Such solutions are especially disadvantageous in covert military situations where minimizing size and transmit power and maximizing stealth are of utmost importance. Accordingly, it would be highly desirable in power-limited scenarios to enhance range performance without resorting to such solutions.